So, in plain english, is there a pro reason that one would want to split audio and video?

You have more control (read quality control) over the audio signal.

Sirc's brilliant retort of "running HDMI to the t.v. then running the audio from the t.v. output" (so you can run better sounding preamps, amps and speakers than what is built into the television) is actually a way to split the video and audio. (The audio outputs obvioulsy do not send any video information...) If you don't run the audio through something else you're stuck listening to those tiny POS speakers that come with the set.

The goober agreed with me and he's too dumb to realise it. Honestly, I don't even know what's he going on about now.